1. Field of the Invention
This invention is related in general to nano-dielectric materials with plasmon-resonance electric-field effects tailored to enhance optical and dielectric properties. In particular, the invention pertains to a multilayer composite wherein three dimensional matrices of uniformly distributed nanoparticles are embedded between alternating continuous polymeric dielectric films.
2. Description of the Related Art
Electric energy storage devices, photovoltaics, displays, biosensors and a multitude of photonic devices could benefit greatly from advanced nano-dielectric materials that are tunable for particular electronic and optical applications. In general, nano-dielectric materials are evaluated for different performance characteristics of interest in various segments of the electromagnetic spectrum. For example, at low frequencies (1 Hz-1 MHz), the insulation properties of the material are important as they relate to capacitor, cable, transformer and other such applications. At higher frequencies (GHz and THz), the dielectric constant, dissipation factor, leakage current, breakdown strength and surface flashover are of particular interest for microwave and pulse power applications. At infra red, visible and UV spectra, the optical properties of the material, such as transmission, absorption and refractive index, are used to characterize its properties.
The present invention is focused mainly on tunable nanocomposite materials with improved energy storage and optical properties. With regard to capacitors, polymer dielectrics such as epoxies and other polymer chemistries have been mixed with both conductive and insulating nanoparticles to produce composites with high dielectric constant. A summary of such prior art is reported by Jiongxin Lu et al. in “Recent Advances in High-k Nanocomposite Materials for Embedded Capacitor Applications,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 15, No. 5, October 2008, pp. 1322-1328. Most such materials are formulated on the basis of percolation theory. That is, the nanoparticle filler is added to the polymer binder until the nanoparticles are virtually touching. At this stage of particle concentration, the dielectric constant of the composite becomes very high; however, the dissipation factor, the leakage current and the breakdown strength are compromised by the large clusters of agglomerated particles that short out segments of the dielectric. This renders the high k material virtually unusable for high voltage dielectric applications.
Similar composites have been made with other types of nanoparticles. For example, U.S. Pat. No. 6,762,237 describes a material where carbon nanotubes are mixed with a polymer dielectric to enhance the dielectric constant. Such nanocomposite materials, where the nanoparticles are randomly mixed to the percolation limit in a polymeric dielectric, end-up with higher dielectric constants but also with a dielectric strength that is significantly lower than that of the polymer dielectric alone with equal dielectric thickness. This can be useful, for instance, for capacitors used in low voltage applications. However, higher voltage applications (e.g., 100V-1000V or higher) require that a large number of capacitors be connected in a series configuration, which is not practical because the cumulative series resistance and losses become prohibitively high.
With regard to photonic applications, polymer nano-dielectric composites have been used for optical filters, photovoltaic cells, and various linear and non linear photonic devices. The literature describes composite materials that are either coated with conducting or semiconducting nanoparticles or contain nanoparticles in a host material, such as a liquid electrolyte or an insulating polymer dielectric. U.S. Pat. No. 7,486,400 (Saito) teaches a multilayer structure where conductive nanoparticle layers are stacked with alternating layers of dielectric SiO2 particles. Saito teaches that the behavior of such a multilayer structure can be controlled advantageously by alternating metal nanoparticles with dielectric nanoparticles and by selecting the size, horizontal density and vertical distance between them to increase the plasmon resonance effect (as measured by the absorbance of the material). Such structure, where particles are stacked to form a nanocomposite, can take advantage of the plasmon resonance effects in the optical part of the electromagnetic spectrum, but it is not applicable to lower frequency dielectric applications because materials that have gas inclusions in them are not appropriate for high strength dielectrics for capacitor, cables and transformer applications.
This invention is directed at producing nano-dielectric materials that have a precise multilayer structure where the size of the metal, semiconducting or insulating nanoparticles and the distance between them are accurately controlled to produce a multilayer three-dimensional structure that has tens to thousands of two dimensional nanoparticle layers accurately spaced in a void-free polymer medium. Unlike other composite systems that are loaded with conductive nanoparticles to the percolation limit, the distance from one nanoparticle layer of the invention to the next is accurately controlled by the leveling effect and the thickness of the polymer film deposited between them. As a result, the dielectric constant as well as the breakdown strength of the multilayer structure of the invention can be controlled and tailored to obtain optimal specifications for different energy-storage applications. In addition, the material of the invention exhibits tunable electro-optical properties available for a variety of electrical and optical applications for which nano-structured dielectrics have been heretofore unsuitable.